I have a 14x sectional enlargement in front of me, a portrait of a young woman. Although the detail I enlarged was very close to the edge of the 6x6 frame, it shows a little bit more detail on her skin than she'd like to see (I used Delta 100). It's not über-sharp, but surprisingly good. I used a Rolleicord with the more simple Xenar at f/5,6, not wide open, but considering the wide-open performance of a Tessar/Xenar is worse than that of a Planar/Xenotar, I'd expect the sharper lenses to produce at least the equal result wide-open as the Xenar stopped down a bit.
However, if you want to print to 40x40 inches, this is an 18x enlargement from MF. Excellent results can be achieved this way, no doubt, but for a really punchy result, you might want to consider LF.